1. Field of the Invention
The present invention relates to an ultrasonic flowmeter, for use in fluid transportation in various industries of chemical plants, semiconductor manufacture field, food processing field and biotechnology field, etc., which propagates ultrasonic vibration through a fluid and measures the flow velocity and flow rate of the fluid from the difference between the ultrasonic wave propagation time from the upstream side and the ultrasonic wave propagation time from the downstream side of the fluid flow, or more specifically, to an ultrasonic flowmeter suitable for measuring a micro flow rate and the flow rate of a slurry fluid or especially a CMP slurry fluid used in the semiconductor field.
2. Description of the Related Art
A conventional ultrasonic flowmeter, as disclosed, for example, in Japanese Unexamined Patent Publication No. 2006-208159, as shown in FIG. 8, includes two openings 53a, 53b formed, in opposed relation to each other at two positions spaced apart from each other in the direction of the fluid flow in a pipe member 51, on a pipe wall 52 of a pipe member 51 through which the fluid flows, and two ultrasonic transceivers 54a, 54b disposed in two openings 53a, 53b, respectively, and used for measuring the flow rate of the fluid flowing through pipe member 51. Two ultrasonic transceivers 54a, 54b are located outside the boundary with the inner peripheral surface of pipe wall 52 of pipe member 51 in corresponding openings 53a, 53b, respectively. The gap between each of ultrasonic transceivers 54a, 54b in openings 53a, 53b and the corresponding boundary with the inner peripheral surface of pipe wall 52 of pipe member 51 is filled with an ultrasonic propagation substance 55 so as not to cause any disturbance of the fluid flowing through pipe member 51 at the boundary of each of two openings 53a, 53b with the inner peripheral surface of pipe wall 52 of pipe member 51. In this conventional ultrasonic flowmeter, it is easy to position two ultrasonic transceivers 54a, 54b relative to pipe member 51 or mount/demount two ultrasonic transceivers 54a, 54b to/from pipe member 51, and the flow rate of the fluid flowing through pipe member 51 can be measured with high accuracy.
However, in this conventional ultrasonic flowmeter, ultrasonic transceivers 54a, 54b are disposed in opposed relation to each other at two positions spaced apart from each other in the direction of the fluid flow, and the propagation distance of the ultrasonic wave is naturally short. Therefore, a problem occurs in which the accuracy of flow rate measurement is not high. Further, an attempt to secure a long propagation distance of the ultrasonic wave would require a larger diameter of the pipe member 51 than a predetermined value, and therefore, a problem is posed that the conventional ultrasonic flowmeter is not suitable for measurement of the flow rate of a fluid flowing through a small-bore pipe. Another problems are that since the positions of ultrasonic transceivers 54a, 54b are fixed, the measurement accuracy cannot be adjusted appropriately as the ultrasonic flowmeters can be arranged only in a predetermined space in the pipe and that the structure is so complicated that the assembly work is bothersome.